CL--DEPENDENT PHOTOVOLTAGE RESPONSES OF BACTERIORHODOPSIN - COMPARISON OF THE D85T AND D85S MUTANTS AND WILD-TYPE ACID PURPLE FORM

Laser flash-induced photovoltage responses of the D85S and D85T mutant s as well as of the wild-type acid blue form are similar and reflect i ntraprotein charge redistribution caused by retinal isomerization. The Cl--induced transition of all of these blue forms into purple ones is accompanied by the appearance of electrogenic stages, which is probab ly associated with Cl- translocation in the cytoplasmic direction. Cl- translocation efficiency of these purple forms is much lower than tha t of the proton transport by the wild-type bacteriorhodopsin. The valu es of the efficiency do not exceed 15, 8 and 3% for the D85T, D85S and wild-type acid purple form, respectively, Cl- induces an additional e lectrogenic phase in the photovoltage responses of the D85S mutant and the wild-type acid purple form. This phase is supposed to be associat ed with the reversible Cl- movement in the extracellular direction. It is interesting that this component is absent in the photovoltage resp onse of the D85T mutant which has, like halorhodopsin, a threonine res idue at position 85. (C) 1997 Federation of European Biochemical Socie ties.